How Would Humans Know If They Lived in a Multiverse?

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Some theories in physics give rise to the idea of multiple
universes, where nearly identical versions of the known universe
exist. But if such a multiverse does exist, how would people
know, and what would it mean for humanity?

There may be ways to find out if the known universe is one of
many, said Brian Greene, a theoretical physicist and author at
Columbia University in New York.

For example, in the
multiverse suggested by string theory, a model that says the
universe is composed of one-dimensional strings, the known
universe might exist on a giant 3D membrane, Greene told Live
Science.

In such a world, "if the universe is a loaf of bread, everything
we know about takes place on one slice," he said. Conceivably,
debris from collisions that migrated off our slice into the wider
cosmos might leave missing energy signatures, which a particle
accelerator like the
Large Hadron Collider at CERN might be able to detect, Greene
said.

Some theories of inflation, the notion that the universe expanded
rapidly in the first fractions of a second after the Big Bang,
suggest
another kind of multiverse. The Big Bang could be one of many
big bangs, each giving rise to its own universe — a cosmic bubble
in a sea of other bubbles.

In such a scenario, the known universe might collide with another
one, which might leave an imprint on the cosmic microwave
background, the radiation signature left over from the Big Bang,
Greene said.

Greene stressed that all of these notions are highly speculative
— "There's reason to take the ideas seriously, but they are far
from science fact," he said.

Is free will dead?

But if a multiverse does exist, it could have some wacky
consequences. A world with an infinite number of universes would
virtually ensure that conditions in one universe would repeat in
another, Greene said. In other words, there would almost
certainly be another version of you reading this article, written
by another version of me.

In such a multiverse, you might decide to read the article in one
universe and not read it in another. What would that mean for the
notion of free will ?

Scientific equations describe the particles that make up all
matter, including humans, Greene said. While more-complex
structures arise that have no relevance to a single particle —
temperature, for instance — everything still has a "fundamental
microphysical underpinning," he said.

That means free will is merely a human sensation, not actual
control.

"When I move my teapot, that sensation is absolutely real," Green
said. "But that's all it is. It's a sensation."

Maybe in another universe, there's a Brian Greene that believes
in free will.